Downhole rotary pumps, such as progressing cavity pumps, are used for the conveying of different types of liquids, but are especially well suited for the pumping of very viscous or thick liquids such as crude oil admixed with a large portion of sand. A downhole rotary pump is driven by a drive string, generally consistent of a rod or tube string having a polished rod at its upper end. The drive string rotates in a stationary production string and is suspended from, and rotated by a drive head assembly associated with the wellhead. The drive head assembly must be able to suspend the weight of the drive string which can become quite substantial for deep wells, while allowing it to be rotated in the production string. Furthermore, the drive head must include a structure permitting the transmission of sufficient torque to the drive string for the pumping of thick liquids.
A drive head assembly for use with rotary downhole pumps is disclosed in U.S. Pat. No. 4,372,379 by Kulhanek et al. The assembly includes a hollow spindle through which the drive string may extend, a frame supporting the spindle on a wellhead and a sprocket connected with a motor for rotation of the spindle. The drive string is suspended from the spindle by a polished rod clamp which rests on top of the spindle. The polished rod is of rectangular shape and is received in a complementary recess in the top end of the spindle. This dog clutch arrangement permits the transmission of torque from the spindle to the drive string. Although this drive head arrangement performs the desired functions of suspending and rotating the drive string, there are certain problems associated with its operation. The polished rod clamp, which is designed to hold but not rotate a drive string, may slip on the polished rod at high torque. As a result, the drive string may slide down through the spindle, which may lead to serious damage of the downhole pump. Furthermore, the drive string cannot be exactly centered in the spindle, since the bore of the spindle must be sufficiently larger than the polished rod diameter to permit insertion of the polished rod in the field and the fit of the rectangular polished rod clamp in the recess of the spindle generally cannot be made close enough to avoid radial movement of the clamp in the recess. However, insufficient centering of the polished rod results in polished rod wobble which is the principle cause of stuffing box leakage and has been a persistent problem for the industry.
Thus, a drive head assembly is desired which substantially prevents polished rod wobble and drive string slippage.